The present invention relates to amplifiers and in particular amplifiers which are of a push-pull type.
Many power amplifiers are of a push-pull type. Push pull denotes that load current is shared between output devices in the push pull amplifier. The load current is shared so that one or more output devices on an upper or positive side of a common or zero voltage reference (hereinafter referred to as xe2x80x9cupper devicexe2x80x9d) sources the load current when the drive signal is positive in polarity, and one or more output devices on a lower or negative side of the common voltage reference (hereinafter referred to as xe2x80x9clower devicexe2x80x9d), sinks the load current when the drive signal is negative in polarity.
In amplifier designs for audio applications high fidelity is often of paramount importance. One factor affecting fidelity is distortion and in particular transient distortion for assymetric signals. Crossover distortion in particular has long been recognised as a difficult problem. Various techniques have been used to reduce crossover distortion and some manufacturers claim distortion levels of 0.0002% or better. Low distortion is a differentiator in the market place for high fidelity amplifiers that separates the best products from competing ones.
A traditional approach to reducing crossover distortion in amplifiers has been to increase bias current. This reduces the risk that output devices will turn off and contribute to crossover distortion and moves the transfer function to a steeper slope region of the output devices thus improving available gain. However, such an option is not viable with integrated circuit power amplifiers because of heat dissipation constraints.
Recent advances in the art make use of the orthogonal property of the bias current with respect to the load current to allow a continuously varying bias current instead of a steady bias current. The orthogonal property refers to the condition wherein the load current will not change if the upper and lower device currents are changed or xe2x80x9cpushedxe2x80x9d simultaneously by the same amount as happens, for example, when a steady bias current is increased. This occurs because the load current in a push-pull amplifier is the difference between the upper and lower device currents. Hence the bias or xe2x80x9cpush-pushxe2x80x9d current is invisible to the load or xe2x80x9cpush-pullxe2x80x9d current. The prior art includes varying the bias current instantaneously with the signal by means of a supplementary internal feedback loop which attempts to maintain conduction of upper and lower output devices at all times. A non-linear control function is used to produce a somewhat gradual transition to the minimum conduction or bias state. The non-linear control function provides a practical means of migrating from load current to minimum conduction condition in the output devices but without regard to the harmonic frequencies generated in the currents of the output devices. Using this approach distortion figures are typically around 0.01%. To be competitive with the best designs a 30 fold improvement in distortion figures is required. It is anticipated that, applied properly, the approach of the present invention has the potential to provide at least a 30 fold improvement.
In the prior art it has been assumed that a diode junction or emitter base junction provides an acceptable non-linear control function. Fundamentally, the present invention lies in a recognition of the need to control harmonic frequencies generated by transitions from load current to minimum bias current in the output devices of an amplifier. An amplifier according to the present invention may include a bias control circuit for controlling the harmonic frequencies which are generated during the transitions. The bias control circuit may be incorporated in a negative feedback loop to dynamically control the bias current. The feedback loop may be combined with an appropriate non-linear control function. The present invention requires that harmonic frequencies generated in the currents of its output devices during the transitions be within the capability of the amplifier. The harmonics generated by the transitions from load to minimum bias current should be controlled to be within the frequency response of the amplifier and to be within the amplifier""s capability to maintain adequate loop gain during the transitions. For a given idle bias current setting, adequate loop gain may be achieved by controlling the harmonic frequencies generated during the transitions from load to minimum bias current.
Harmonic frequencies may be controlled by shaping the non-linear characteristic of the control function. Characteristics of the control function may be shaped by introducing a linearity control circuit in combination with a non-linear transform circuit to moderate relative linearity of the control function. The linearity control circuit may include at least one shaping element such as a resistor and may be situated such that it creates a voltage drop which is sensed as part of the input signal to the non-linear transform circuit. The linearity control circuit may contain a network of components in the form of a shaping circuit to provide a required transfer function in the bias feedback loop to control the harmonic frequencies. The network may include one or more resistors, diodes and transistors.
According to the present invention there is provided a push pull amplifier having upper and lower output devices operating in a mode in which said devices drive a load alternately, said amplifier including bias means for providing a bias current to said devices at all times, said bias means being incorporated in a feedback loop and being arranged such that transitions from load current to minimum bias current in said upper and lower output devices are sufficiently gradual so that harmonic frequencies generated by said transitions are within the capability of said amplifier under all signal conditions, said feedback loop including non-linear transform means for each said upper and lower output device to prevent said bias current reducing to zero and linearity control means for controlling the harmonic frequencies in said upper and lower output devices.